Of various types of techniques for detecting torsional amounts of two relatively-rotatable shafts, there have been well known those which are characterized by provision of detection devices, such as a potentiometer or resolver devices, on input and output shafts interconnected via a torsion bar. According to the above-mentioned technique using a potentiometer, a slider is mounted on the input shaft while a resistor is mounted on the output shaft, so that a position of the slider contacting the resistor varies in accordance with variation in a relative rotational position between the input and output shafts to thereby provide an analog voltage corresponding to the relative rotational position. According to the technique using resolver devices, separate resolver devices are provided on both of the input and output shafts so as to detect a relative rotational amount (torsional amount) between the two shafts on the basis of angle signals output from the two resolver devices. Further, as a means for detecting a relative rotational displacement between two relatively-rotatable shafts, there has been developed a noncontact-type torque sensor for electronic power steering which employs an induction coil (see Japanese Patent Application Laid-open Publication No. HEI-6-174570).
Inventions disclosed in Japanese Patent Application Laid-open Publication Nos. 2000-55610, 2002-48508, 2002-107110 and 2002-310816 were each proposed with a view to avoiding the drawbacks of the above-discussed prior art, and each of these proposed inventions employs a noncontact-type and induction- or variable-magnetic-coupling-type sensor that uses a coil section and magnetism-responsive member (magnetic substance or electrically-conductive substance). Each of these inventions is suited for accurate detection by a phase detection scheme.
The conventional technique of the type using a potentiometer would always suffer from poor electrical contact, failure and/or other problem since the electrical contact is implemented via a mechanical contact structure. Further, because there occurs impedance variation due to temperature changes, it is necessary to appropriately carry out temperature drift compensation. Further, the conventional rotational-displacement detection apparatus, known as the noncontact-type torque sensor for electric power steering employing the induction coil, is arranged to measure an analog voltage level produced in response to a minute relative rotational displacement, so that it would accomplish only a very poor detecting resolution. Further, in addition to the need to compensate temperature drift characteristics of the coil, there is a need to appropriately compensate temperature drift characteristics present in reluctance of magnetic substances that vary magnetic coupling to the coil in response to a changing relative rotational position as well as in eddy current loss of electrically-conductive substances. Furthermore, for application to a torque sensor, it is necessary to precisely detect a minute rotational angle, and thus, with the detection apparatus using a noncontact-type and induction- or variable-magnetic-coupling-type sensor as disclosed in any one of the above-identified patent literatures, it is desirable to make arrangements for providing sufficient output voltage levels of the sensor coil.